Color television systems have been developed using several different broadcast and signal processing formats to achieve the successful transmission and reception of color television programming. While substantial differences between systems exist, they all must satisfy the basic objective of combining the picture or luminance information, the color or chrominance information, and sound information together with appropriate display scan synchronizing signals to form an information signal which may be modulated upon a carrier for transmission. At the receiver, the opposite processes must take place in which the several components of the information signal are separated and appropriately processed. In most television broadcast formats such as the NTSC system used within the United States of America and the PAL system used in many European countries, the signal components corresponding to luminance, chrominance and sound are distinguished from each other and separated for individual processing largely on the basis of signal frequencies. For example, in the NTSC system, the available broadcast bandwidth is maintained at 6 megahertz. To conserve channel bandwidth and to transmit up to 4.1 megahertz of video signal, a vestigial sideband format in which the carrier is off center within the 6 megahertz channel bandwidth is used. The chrominance information is modulated upon a chrominance subcarrier separated from the picture carrier by approximately 3.58 megahertz. The sound information is separated from the picture carrier by 4.5 megahertz. To further conserve channel bandwidth, the luminance signal and chrominance signal share a part of the channel bandwidth.
Thus, a low cost receiver is able to select the chrominance, sound and luminance signal portions by using appropriate frequency response networks or filters and thereafter perform individual processing thereon. Unfortunately, the frequency selection process used in most television receivers results in the loss of substantial amounts of information or image content. Perhaps the most notable loss occurs in the video or luminance information which is severely bandwidth limited as a result of the separation of chrominance and sound information. While these losses have been recognized as less than desirable, the basic filtering processes used in most television receivers has made improvement difficult or impractical. The loss of higher frequency luminance information results directly in a reduction of the resolution in the displayed image. In seeking to maintain image resolution by preserving high frequency luminance information, practitioners have attempted to extract the luminance information which is found in the chrominance band. For example, practitioners have used analog glass delay line comb filters to separate luminance and chrominance information from the shared frequency spectrum while preserving high frequency luminance. It has been determined, however, that glass delay lines do not provide accurate delay and factory alignments are usually required to accurately separate luminance and chrominance signals.
Another approach contemplated by practitioners in the art seeking to improve the recovery of information at the receiver is found in the use of digital signal processing rather than the more pervasive presently used analog signal processing. Several advantages are provided by digital signal processing. For example, the separation of chrominance and luminance information in a digital environment may be carried forward using comb filters which use accurate delay and therefore accurately separate the luminance and the chrominance signals. Effective comb filters are more easily realized in the digital environment. In addition, a variety of information processing techniques which require memory for temporary storage of information are facilitated in a digital environment due to the ease with which memory may be achieved. Similarly, signal delays are relatively easy to perform in a digital signal environment. More generally, digital systems have evolved to a level of sophistication in which many digital systems have proven to be more economical to manufacture and have required fewer adjustments than their corresponding analog systems.
Despite the promise of advantages of the type set forth above to be realized by the application of digital electronic processing of television receiver signals, several problems and limitations have also arisen. For example, a fundamental bandwidth limitation is imposed upon digital processing circuits by the sample or clock rate which the system uses. Generally speaking, the sample or clock rate must be at least twice as large as the highest frequency signal component being processed. Unfortunately, increased sample or clock rates often results in dramatically increased system complexity which in turn increases costs. In addition, in broadcast formats such as the above-mentioned NTSC or PAL systems, the received information is analog information and thus the use of digital circuit processing thereon requires that the signals be converted from analog-to-digital signals. In most instances, the analog-to-digital conversion circuits used can only convert lower frequency components of the analog signal due to the practical limits of the sample rate. To convert higher frequency components of the analog signal requires more complex converters operated at high sample frequencies and more expensive delay elements because more samples have to be delayed. The results obtained in image resolution through maximizing the luminance information bandwidth generally fall short of the desired objective. Television images frequently lack a high resolution appearance despite improved luminance bandwidth.
Thus, even though practitioners in the art attempt to maximize the resolution of the displayed image, practitioners have for many years also employed an additional technique known as luminance or video peaking. Video peaking is the process of increasing the relative amplitude of luminance signal components having frequencies corresponding to the luminance transitions or image element "edges". It has been established by experience that luminance transitions which are rich in harmonics of approximately two and one half megahertz provide sharp images. Video peaking emphasizes these signal components and increases the "sharpness" or "crispness" of the displayed image.
In the typical peaking system presently used, the frequency response of the luminance signal amplifiers driving the display are increased or peaked at or near the frequencies of transition harmonics. Because the degree of video peaking desired varies among different viewers, a viewer accessible control is usually provided to accommodate viewer preferences.
Unfortunately, the effect realized by such video peaking systems is often erratic and extremely sensitive to broadcast signal variations and scene content changes. In addition, video peaking systems tend to be extremely interactive with the remainder of the luminance system often disturbing the luminance signal processing.
There remains, therefore, a need in the art for a video peaking system which may be independently controlled and which compensates for changes of broadcast signal conditions.
Accordingly, it is a general object of the present invention to provide an improved video processor. It is a more particular object of the present invention to provide an improved video processor which effectively peaks the luminance signal transition components within a composite video signal while providing independent control of video peaking and automatic compensation for signal variations.